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Gear oil is one of the common lubricants used in various gear transmission systems to prevent tooth surface wear, scuffing, sintering, etc., thereby extending their service life and improving the efficiency of power transmission. The six key functions of gear oil as summarized by experts in gear oil testing: Testing parameters for gear oil include kinematic viscosity, viscosity index, water content, total acid value, pour point, flash point, spectral metal element analysis, antifoaming properties, extreme pressure performance, PQ wear index, and ferrography wear analysis. Common physical and chemical analysis concepts, methods, and purposes for gear oil testing: (1) Viscosity – Basic concept: Viscosity is a measure of the internal friction force when a fluid flows, and it is used to determine a gear oil’s ability to resist flow at a specific temperature. Testing method: A capillary viscometer is used to measure the dynamic viscosity of gear oil. GB/T 265, ASTM D445 – Purposes of gear oil testing: The main basis for classifying gear oil grades, the primary criterion for testing and selecting gear oil, important indicators of gear oil degradation, and used to determine whether the appropriate oil is being used. (2) Water content – Basic concept: Refers to the percentage of water contained in the oil (free water, emulsified water, dissolved water). Testing method for gear oil: Distillation is used for testing gear oil. Purpose of testing according to GB/T 260 and ASTM D95: Moisture destroys the oil film, reduces lubricity, and accelerates wear of the frictional components. It can also react with gear oil to form acids, gums, and sludge; these substances can precipitate out of the gear oil, thereby reducing its performance. At low temperatures, moisture impairs the fluidity of the oil and causes corrosion and rusting of the metal components in the equipment. (3) Flash point: Basic concept – the lowest temperature at which vapor from gear oil begins to ignite under specified heating conditions. Testing method: ASTM D92, GB/T 267. Purpose of testing: Testing the flash point of gear oil allows for determining the lightness or heaviness of the composition of its fractions ； The flash point is a safety indicator for gear oil ； The flash point can detect the presence of light fuel oils mixed in gear oil. (4) Total acid value: Basic concept – The amount of acid required to neutralize all acidic components in 1 g of the sample, converted into an equivalent amount of acid, expressed in mgKOH/g. Detection methods: color indicator method and potentiometric titration method. Purpose of GB/T 7304 and ASTM D664 gear oil testing: to determine the degree of refinement of the base oil ； Measurement of acidic additives in refined petroleum products ； An important indicator for determining the oxidative degradation of gear oil during use. (5) Total alkalinity: Basic concept – The amount of acid required to neutralize all the basic components in 1 g of the sample, converted into an equivalent amount of alkali, expressed in mgKOH/g. Detection method: perchloric acid potentiometric titration. SH/T0251-1993, ASTMD2896 – Purpose of testing: To determine the amount of basic detergency and dispersancy additives present in diesel engine oils. Monitoring the ability of alkaline additives to prevent oil oxidation, testing the total alkalinity value of new oil. (6) Contamination analysis: Basic concepts – detecting the size, quantity, and distribution of contaminant particles in the oil. Detection method: Automatic particle counting method (light-blocking method). Purpose of NAS 1638 and ISO 4406 gear oil testing: to quantitatively determine the quantity of contaminant particles in gear oil as well as the level of contamination ； In precision hydraulic systems, solid particle contamination will accelerate the wear of control components ； In turbine systems, solid particle contamination will exacerbate the wear of components such as bearings